[composer] Ensure that composers are deleted before compositions

Fixes some rare crashes when loading projects.

Author: nyalldawson

src/app/qgisapp.cpp

@@ -5465,8 +5465,19 @@ void QgisApp::deletePrintComposers()
   for ( ; it != mPrintComposers.end(); ++it )
   {
     emit composerWillBeRemoved(( *it )->view() );
-    delete(( *it )->composition() );
+
+    //save a reference to the composition
+    QgsComposition* composition = ( *it )->composition();
+
+    //first, delete the composer. This must occur before deleting the composition as some of the cleanup code in
+    //composer or in composer item widgets may require the composition to still be around
     delete( *it );
+
+    //next, delete the composition
+    if ( composition )
+    {
+      delete composition;
+    }
   }
   mPrintComposers.clear();
   mLastComposerId = 0;